Development of a Two-way Nested LETKF System for Cloud-resolving Model - Data Assimilation for Atmospheric, Oceanic and Hydrologic Applications - page 482

improve the rainfall forecast. A number of data assimilation experiments on radial
wind from Doppler radars and GPS-PWV using the EnKFs have been reported so
far (e.g. Xue et al. 2006 ; Seko et al. 2011 ). The impacts of the GPS-SWV and the
synergistic effect of simultaneous assimilation of Doppler radar data and GPS-water
vapor data (PWV or SWV) have not been investigated so far with EnKFs, though
they were shown in the data assimilation experiments in which the JMA meso-4dvar
assimilation system was used (e.g. Seko et al. 2004 ). In this study, these impacts are
investigated with EnKFs, in addition to the impacts of GPS-PWV data and Doppler
radar data.
In Sect. 20.2 , a local heavy rainfall which the nested LETKF system was applied
to is explained. Section 20.3 briefly explains the nested LETKF system. Results of
assimilation of conventional data are described in Sect. 20.4 . Impacts of GPS data
and Doppler radar and their synergistic effect are shown in Sect. 20.5 . Section 20.6
is the conclusion of this study.

20.2 Thunderstorm of 5th September 2008 Developed

on the Osaka Plain

As the target of experiments in this study, a local heavy rainfall generated on the
Osaka Plain on 5th September 2008 was adopted, because Doppler radar data of the
Osaka and Kansai international airports can be used.
On 5th September 2008, a high pressure system widely covered Japan and a
small low pressure system was seen in the south of western Japan (Fig. 20.1 a).
Because large-scale disturbances, such as synoptic fronts or typhoons did not exist
near Japan's main islands, scattered convection cells were generated not only near
Osaka, but also over western Japan (Fig. 20.1 b). Figure 20.2 b shows the radar
echo distribution observed by conventional radars of JMA (Japan Meteorological
Agency) from 1400 JST to 1600 JST (Japan Standard Time; 0900 JST corresponds
to 0000 UTC; Universal Time Coordinate). At 1400 JST, there were scattered
convection cells in mountainous areas east and south of the Osaka Plain (indicated
by circles in Fig. 20.2 a). These convection cells developed there by 1500 JST and
other intense convection cells were generated on the Osaka Plain. The intense
convection cells on the Osaka Plain organized, and then produced a rainfall amount
exceeding 90 mm from 1450 JST to 1600 JST at Sakai City (not shown). This
rainfall region extended northwestward, maintaining its rainfall intensity until 1600
JST. The horizontal wind and the sea level temperature and pressure at 1400
JST are shown in Fig. 20.3 . A high temperature region existed on the Osaka
Plain (Fig. 20.3 b) and a thermodynamic low-pressure system was generated there
(Fig. 20.3 c). The southerly flow from the Kii-channel and the northeasterly flow
over the Osaka Plain were converged near Sakai City (Fig. 20.3 a). This low-level
convergence is one of the reasons why the intense convection cells generated on the
Osaka Plain.